More precisely, the current state of the art is as follows:
Polished metal surfaces such as copper, molybdenum or stainless steel used as mirrors are reflective in the infrared but do not have a sufficiently high degree of reflectance for the desired uses, with the desired reflectance being not less than 0.995. The reflectances of these materials are situated respectively at about 0.99, 0.98 and 0.90 at the wavelength under consideration. This may be seen from the article "Pulsed CO.sub.2 laser damage in windows, reflectors, and coatings" by Wang, Rudisill, Giuliano, Braunstein and Braunstein, published in NBS special publication No. 414 entitled "Laser induced damage in optical material". The three above-mentioned materials have good high-power performance, but copper is a soft material and in this respect has inadequate mechanical strength.
The reflectance is increased by putting a layer of silver or gold on the metal support. In the infrared, these two metals have higher reflectances (0.993 and 0.988) and their high-power thresholds are high. The methods most commonly used for making such surfaces include vacuum evaporation or cathode sputtering. Another advantage of using these metals lies in the fact that non-metallic materials can then be used as supports. Mention may be made of silicon, for example, which even though it has a low coefficient of reflection (R=0.30), becomes usable once the thickness of the deposited metal layer is large enough for the layer to be considered as being solid. This condition is achieved when the thickness of the layer is approximately a few hundredths of a wavelength. A thickness of 150 nm is often enough. However, silver has poor chemical performance in the environment since it has very high affinity for sulfur; in addition, neither gold nor silver is hard enough for providing good mechanical properties.
A conventional method currently in use for increasing reflectance consists in making a stack of dielectric layers alternating between a high index n.sub.H and a low index n.sub.B such that their optical thicknesses n.sub.H e.sub.H and n.sub.B e.sub.B over the support which is already covered in gold or silver (where e.sub.H and e.sub.B refer to the geometrical thicknesses of the layers) are one quarter of a wavelength or thereabout. The reflectance of a metal layer in terms of its complex index n-ik (where i.sup.2 =-1, n is its refractive index, and k is its extinction index) is given by the relationship: ##EQU1##
With a stack of 2p transparent dielectric layers, i.e. layers having substantially zero extinction indices, the reflectance increases to the value: ##EQU2## with a layer of index n.sub.B being deposited on the metal layer. In order for the reflectance to be as high as possible with a given number of layers, it is desirable for the index of the low index material to be as low as possible and the index of the high index material to be as high as possible.
In practice, dielectrics always absorb at least a little (their extinction indices k.sub.H and k.sub.B are not exactly zero). The limiting reflectance then no longer tends towards 1 as the number of layers increases indefinitely, but towards a limiting value given by Koppelmann: ##EQU3## It can thus be seen that in order to obtain high reflectance values, it is necessary for the selected materials to absorb very little (k.sub.H and k.sub.B being not more than a few 10.sup.-4).
By way of example, FIG. 1 shows a deposit made on a molybdenum support 11 having deposited successively thereon by vacuum evaporation: a layer of silver 12; followed by two pairs of quarterwave dielectric layers, each pair comprising one layer of thorium fluoride (13 or 15) having an index 1.35 and one layer of zinc sulfide (14 or 16) having an index 2.2. This type of coating has a reflectance of 0.996 and its high-power performance limit is high. However, since the majority of materials which are transparent in the vicinity of a wavelength of 10.6 micrometers are not very hard, the final deposit has inadequate mechanical performance.
An improvement can be obtained by depositing a fine hard layer of a protective dielectric material such as a fluoride (MgF.sub.2, ThF.sub.4, CeF.sub.3 or LaF.sub.3) or an oxide (TiO.sub.2, ZrO.sub.2, CeO.sub.2, HfO.sub.2, Y.sub.2 O.sub.3) onto the silver or the gold or onto the stack of dielectric layers. Under such conditions, the stack or the gold or the silver are better protected both mechanically and chemically. However two major defects remain:
the mechanical and chemical protection necessarily gives rise to reduced reflectance. This reduction increases with increasing index of the selected protective material, with increasing thickness of the protective layer, and with increasing infrared absorption therein; and
the structure of the deposited dielectric layer generally includes holes which are distributed in columns. These holes are inherent to the manufacturing process. They fill with water vapor when the deposit is placed in air and prevent effective chemical protection against external agents from being obtained. This subject is described in the article entitled "Inhomogeneous interface laser mirror coatings" by A.M. Ledger appearing at pages 2979-2989 of the Sept. 1, 1979 number of Applied Optics vol. 18, No. 17 published by the Optical Society of America, New York, US.
Other protective layers may be obtained using amorphous materials such as carbon having high hardness (3000 kg/mm.sup.2) and very low porosity. However, it is not possible to obtain sufficient reflectance with this material given the residual absorption of the layer and its refractive index which is too high (n about 1.8) for such conditions of use. Reference on this subject may be made to the article "Properties and coating rates of diamond-like carbon films produced by RF glow discharge of hydrocarbon gases" by L.P. Andersson, S. Berg, H. Norstrom, R. Olaison, S. Towta, in "Thin solid films" 63 (1979), pages 155-160.
Although the above-explained problem of reconciling optical qualities and protective qualities is being faced at present in particular with mid infrared radiation, it is clear that it could also arise in the future in other parts of the spectrum.
The present invention has the aim, in particular, of providing a stack of layers making it possible to simultaneously obtain the high reflectance required of a carbon dioxide laser mirror together with good protection against mechanical attack.
Another aim is to provide good protection against atmospheric agents and high high-power limit in a light flux such as that produced by a carbon dioxide power laser.